Charge pumps are used in microelectronics for generating one voltage level from another. Common applications include the boosting or attenuating of a given voltage to some new value.
An ideal charge pump is illustrated in FIGS. 1A and 1B to which reference is now made. Reference is also made to FIG. 2 which is a timing diagram of the control signals for the charge pump of FIGS. 1A and 1B.
The ideal charge pump includes an initial charge transfer switch 10, a plurality of boost stages 12 (FIGS. 1A and 1B shows two, labeled 12a and 12b) and a load capacitor 14 having capacitance CL. The initial charge transfer switch 10 connects an input Vdd voltage source to the first boost stage 12a. Each boost stage includes a boost capacitor 16 having capacitance Cb and a charge transfer switch 18. Boost capacitors 16 are connected between an input voltage source and a clock voltage source (labeled CLK or CLKb, the complementary signal of the CLK signal). Load capacitor 14 is selectively connected in parallel to the last boost capacitor 16b via the last charge transfer switch 18b. As indicated in FIGS. 1A and 1B, the switches 10, 18a and 18b operate alternatively, in accordance with the state of the clock signal.
Each stage operates in two phases: pre-charge and boost. Pre-charge occurs when the associated clock voltage of a boost capacitor 16 is low, its charge transfer switch is open and that of the previous stage is closed. Thus, in FIG. 1A, boost capacitor 16a is in the pre-charge phase. The initial charge transfer switch 10 is closed and switch 18a is open. As arrow 19 indicates, charge is transferred to boost capacitor 16a.
Boost occurs when the associated clock voltage of a boost capacitor is high, its charge transfer switch is closed and that of the previous stage is open. During boost, charge is transferred to the next stage. In FIG. 1A, boost capacitor 16b is in the boost phase, with charge being transferred to the load capacitor 14, as indicated by the arrow 21. In FIG. 1B, the complementary clock levels reverse the capacitor roles. Thus, boost capacitor 16b is in the boost phase and capacitor 16b is in the pre-charge phase, as indicated by arrow 23.
The opening and closing of switches 10 and 18 occur in accordance with the state of the clock signal CLK which, as shown in FIG. 2, is at a low voltage-from t0 to t1 and at a high voltage from t1 to t2. When the clock signal CLK is at a low voltage, the odd numbered switches (e.g. switches 10 and 18b) are closed and the even numbered switches (e.g. switch 18a) are open, as shown in FIG. 1A. As the clock signal CLK transitions to a high voltage, such as the supply voltage Vdd, the odd numbered switches 10 and 18b open and the even numbered switch 18a closes, as shown in FIG. 1B.
Since the CLK signal is connected to one side of the first boost capacitor 16a and since first boost capacitor 16a has charge thereon in FIG. 1B, the high voltage of the CLK signal forces all of the charge stored in the first boost capacitor 16a to be transferred to the second boost capacitor 16b, as shown by arrow 23. As this cycle repeats, more charge is transferred to the second boost capacitor 16b.
Since the capacitance Cb is fixed, increasing the total charge stored therein requires a corresponding voltage increase across the second boost capacitor 16b. The maximum voltage Vmax that can be achieved is the original value (of the supply voltage) boosted by the supply voltage Vdd. In turn, the second boost capacitor 16b (and n total stages) boosts the maximum voltage to be loaded onto the load capacitor 14 by the supply voltage Vdd. Thus, Vmax=n*Vdd.
Ideal switches do not exist. In one prior art embodiment, diodes are utilized for the switches 10 and 18. This is illustrated in FIGS. 3A and 3B to which reference is now made, where FIG. 3A indicates the state corresponding to that shown in FIG. 1A and FIG. 3B indicates the state corresponding to that shown in FIG. 1B. The charge pump of FIGS. 3A and 3B operates in a similar manner to that of FIGS. 1A and 1B.
The diodes, labeled 20, 22a and 22b, allow charge to flow in only one direction, input to output and, in this manner, operate as switches. However, diodes are far from ideal. Charge transfer occurs only if the diodes are biased with a threshold voltage Vdiode of at least 700 mV. The voltage drops of diodes 20 and 22a limit the maximum amount of charge that can be stored, respectively, in boost capacitors 16a and 16b. As a result, the non-idealities imposed by utilizing diodes as switches reduces the amount of "boost" a charge pump can provide. In fact, the maximum boost voltage Vmax is n*(Vdd-Vdiode).
It is known that MOSFETs (metal oxide semiconductor, field effect transistors) can be configured to simulate diode operation. Thus, as shown in FIGS. 4A and 4B (similar to FIGS. 1A and 1B) to which reference is now made, it is known to make a charge pump with MOSFETs 30, 32a and 32b replacing diodes 20, 22a and 22b, respectively, in what is known as a "diode connection". Each MOSFET has a control gate and two input/output diffusions Diff.
The elements are connected together at nodes 34, 36 and 38, where the initial charge transfer switch (provided by MOSFET 30) is connected to the first stage, labeled 40a, at node 34, the first stage 40a is connected to the second stage, labeled 40b, at node 36, and the second stage 40b is connected to the load capacitor 14 at node 38. In the charge pump of FIG. 4, the gate and the input diffusion Diff of each MOSFET 30 or 32 are connected, in a "diode-connection", to the input of the stage (either the Vdd voltage source for MOSFET 30 or at nodes 34 or 36, for the stages 40a and 40b, respectively) and the output diffusion is connected to the output node (node 36 or 38).
When the CLK signal is low, MOSFET 30 is active, passing a voltage to node 34. At the same time, since load capacitor 14 has some voltage on it which is higher than the voltage at node 34, MOSFET 32a shuts off and thus, should not pass any charge. When the CLK signal transitions from low to high, the even and odd numbered MOSFETs switch activates; MOSFET 30 shuts off and MOSFET 32a turns on. As a result, charge stored in capacitor 16a is transferred to capacitor 16b, as indicated by arrow 23.
However, like diodes, MOSFETs also have a threshold voltage, Vth, which is similar or higher to the Vdiode of diodes. MOSFETs therefore, suffer from the same problem as diodes. In other MOSFET devices, the threshold voltage Vth is almost zero or even negative. These "low-threshold" MOSFET devices should make ideal switches. However, in the diode connection configuration of FIG. 4, the MOSFET device cannot strongly turn off and, as a result, there is a back leakage of charge. Therefore, in the prior art, low threshold voltage MOSFETs are not utilized for charge pumps.
For charge pump applications, where the power supply is substantially larger than either Vdiode or Vth, the charge pump configurations described hereinabove are often efficient enough. However, in applications where Vdd is relatively low, for example in today's low-power applications where Vdd is close to Vdiode or Vth, the effect of switch non-idealities renders these charge-pump configurations impractical.